Screen Brightness not Changing on Dell XPS 7590

After installing Ubuntu 18.04 LTS a new Dell XPS 7590 laptop with OLED display, the brightness is stuck at the highest point (the Fn+F11 or F12 commands show that the brightness is being changed, but no brightness change occurs).

I have tried many suggestions from the web, but all failed; e.g., changing the acpi_backlight parameter values in /etc/default/grub, creating an xbacklightmon file, or installing brightness controller.

Is there any way that the brightness could be changed?

I recommend using the ICC Brightness tool, which worked well for my Dell 7590 with OLED screen. It requires compiling and installing a small utility, but it works well.

Full instructions can be found at https://github.com/udifuchs/icc-brightness but here is a summary.

$ sudo apt install git liblcms2-dev
$ mkdir ~/git
$ cd ~/git
$ git clone https://github.com/udifuchs/icc-brightness
$ cd icc-brightness
$ make
$ sudo make install

After a reboot your brightness keys should now work. Be careful not to use this in conjuction with the xrandr solutions as they don't play nicely together.

Enter xrandr and check the output. In my case it was eDP-1. To test if it is the right one, enter xrandr --output eDP-1 --brightness 0.6 (intel driver). If yes, the brightness should have changed.

I found it at How to install Ubuntu on a Dell XPS 15 2019.